This Program Project application will Investigate the molecular mechanisms of prostate cancer initiation. The proposed studies will be pursued by a highly-integrated Interdisciplinary team of investigators who have substantial track records in prostate cancer research, as well as in collaborative interactions with each other. Our proposal has a central focus on the role of NKX3.1, a key regulator of gene expression programs in the prostate epithelium whose expression is down-regulated during prostate cancer initiation. Recent studies by members of our research team have shown that Nkx3.1 is a marker for a prostate epithelial stem cell population that is a cell type of origin for prostate cancer in mice, and that Nkx3.1 plays a role in stem cell maintenance. Furthermore, NKX3.1 participates in the earliest molecular events involved in the response to DNA damage, while NKX3.1 loss predisposes to increased oxidative damage, decreased survival after DNA damage, and cellular senescence. Thus, analysis of NKX3.1 function in prostate tumorigenesis represents an excellent model for investigating cancer initiation. We will pursue three interrelated projects that will investigate the molecular mechanisms of prostate cancer initiation using genetically-engineered mouse and cell culture models, together with essential validation using human xenografts and clinical specimens. Project 1 (Shen) will investigate the potential cell type(s) of origin for prostate cancer in mouse models, and will determine whether these cells of origin can give rise to tumor-initiating cells. Project 2 (Abate-Shen) will analyze the roles for Nkx3.1 in prostate epithelial specification and cellular senescence, and will identify Nkx3.1 target genes through genome-wide analyses coupled with functional validation in mouse and human prostate. Project 3 (Gelmann) will investigate the role of Nkx3.1 in the DNA damage response in cell culture and mouse models. These Projects will be assisted by Core A (Cordon-Cardo), which will provide pathology support for the Projects and will examine the incidence and co-localization of known molecular events in prostate cancer initiation, as well as by Core B (Shen), which will provide administrative, bioinformatic, and biostatistical support for the overall Program.